DA leader Mmusi Maimane has revealed how angry he became when he read Western Cape premier Helen Zille's tweets in defence of colonialism.
Maimane told the Sunday Times he felt offended by the post from his predecessor. He has reported Zille to the party's federal legal commission (FLC) for disciplinary proceedings.
The matter has divided the party as Zille's backers come to her defence while others call for her head.
"I felt angry because I feel that tweets like that - intended or not - are offensive in the context of our history," Maimane told the Sunday Times on Friday.
"In what context could anyone propose that such an offensive system was not all bad? I am deeply disappointed."
In tweets on Thursday, Zille first stated: "For those claiming the legacy of colonialism was ONLY negative, think of our independent judiciary, transport infrastructure, piped water etc." before asking "Would we have had a transition into specialised healthcare and medication without colonial influence? Just be honest, please."
She said she would "cut off those who think EVERY aspect of colonial legacy was bad".
She tweeted an apology after the tweets caused outrage on social media: "I apologise unreservedly for a tweet that may have come across as a defence of colonialism. It was not."
Maimane's comments came as pressure mounted on the DA to remove Zille as premier.
A number of other DA leaders, including provincial heads John Moodey (Gauteng) and James Masango (Mpumalanga), Midvaal mayor Bongani Baloyi and young public representatives such as Mbali Ntuli, Gordon Mackay, Phumzile van Damme and Makashule Gana condemned Zille's tweets.
Asked whether he thought Zille's tweets were racist, Maimane said the issue was before the party's FLC so he did not want to prejudge it.
"I cannot, because I feel offended to the core, suppress freedom of thought. I do believe, though, that Helen Zille's statements are inconsistent with the DA's value of fairness.
"I would argue the effects of colonialism are still well and truly with us. We are still living with it. We must all build an inclusive society. Praising colonialism is not part of the DA I am trying to build," he said.
He said it was unavoidable that people would compare Zille's case to that of party MP Dianne Kohler Barnard, who was fined R20000, removed from all internally elected DA positions and had to pay for public apologies in five newspapers and attend a social media management course at her own expense after reposting a Facebook post praising apartheid president PW Botha.
"I do not want to prejudge the issue and undermine the DA federal legal commission's work by comparing the two cases. Helen Zille has every right to appear and make her case before the [commission], who has to interrogate the issue. It is only natural that comparisons between the two cases will be made," said Maimane.
Former DA leader Tony Leon said Zille's case should be judged in the same way . "The Dianne Kohler Barnard precedent is now the rule of the DA. This case will have to be judged on that criteria," said Leon.
Incidents of racism within the DA prompted the party to request its members to sign a pledge against racial discrimination in January this year.
Maimane said since then the party had dealt harshly with members mobilising on the grounds of race.
"Our values are clear and we have placed great emphasis on diversifying our leadership.
"If the [commission] finds racism to be the charge, Zille's immediate suspension from the DA will follow.
"The DA is winning its war against racism. This fight must be fought everywhere by all South Africans - not just by the DA. The point is that if this threatens the DA and its values, as the leader I will defend the interests of the party."
The chairman of the DA federal executive, James Selfe, said commission chairman Glynnis Breytenbach would investigate whether Zille should be charged and if so, what the charges should be.
Zille on Friday night said she was confident that the FLC would deal with her fairly.
"We follow the rule of law. Our FLC is independent from the political leadership," she said in a text message.